This invention relates to electrodes for rechargeable electrochemical cells such as nickel cadmium cells. In such electrochemical cells a conductive contact area is provided to connect each electrode of the cell to a terminal of the battery in which the cells are placed. Frequently a contact tab is provided on the conductive area to provide a positive connection between the electrode and the corresponding battery terminal.
Electrodes have been made using a foam metal substrate impregnated with an electrochemically active material namely nickel hydroxide paste. On such an electrode it is very difficult to attach a current carrying contact tab. Welding is the preferred method of attachment, although welding such a contact to a foam metal substrate leads to mechanical weaknesses.
Alternatively, electrodes have been made using a perforated metal plate covered with a sintered porous nickel coating. Active material such as nickel hydroxide is then applied to the sintered plaque. Similarly, it is difficult to provide a contact area and to adhere a current conducting contact tab to the sintered plaque loaded with active material. In the past, the current conducting contact tab has been attached to an edge portion of the plate which projects from the portion covered with active material. This reduces the volumetric electrical capacity of the electrode as the electrochemically active ingredient cannot be located where the contact tab is required. Typically, a larger area was left unimpregnated than was required for contact adherence.
U.S. Pat. No. 4,439,281 shows another method of applying a contact tab to a highly porous structural body or substrate. In this patent a metal coating is applied or electrodeposited where a contact tab is desired. This method of electrodeposition of nickel on an edge for contact tab adherence is very time consuming, and the resulting nickel concentration although greater than that for a porous plate substrate, still presents welding problems.
Similarly, Japanese Patent Publication document 61-214363 shows an alternative method of attaching a conducting contact tab to an electrode of a battery. In this example, an energy source is used to fuse the edges of a metal felt. The thus densified metal provides a welding surface for the contact tab. The storage capacity of the cell, however, is diminished as the densified edges are not impregnated with nickel hydroxide for electrochemical activity.
The instant invention selectively converts an area of an electrode to conductive metal to provide a means for improved current collection.
It is well known to reduce nickel hydroxide by using a heated hydrogen source. In the area of electrochemical cells nickel hydroxide has been reduced to provide strengthening characteristics for an electrode. Such is shown in U.S. Pat. No. 1,940,385. However, contact tab attachment and conductive capabilities are not discussed. The conversion of all of the nickel hydroxide to nickel metal is done across the framework rather than in selective areas, for the purpose of achieving greater strength.